1. Field of the Invention
The present invention relates to an etching method that is mainly used in the fabrication of semiconductor devices, a process of fabricating a metal film structure that employs the etching method, and an etching structure that is obtained by the etching method.
2. Description of Related Art
Generally, metal electrodes used in semiconductor devices comprising compound semiconductors are formed by means of the lift-off method.
That is, a photoresist pattern is formed on the principal surface of a substrate having the elements formed thereon. The photoresist pattern has provided therethrough an opening of the same shape as the planar shape of the metal electrode to be formed. Further, a metal of the electrode material is deposited by means of vacuum deposition or the like on all over the surface of the photoresist pattern including the opening. Thereafter, a metal electrode having a predetermined planar shape is residually formed through removal (lift-off) of unused metal film deposited in a region excluding the opening together with the photoresist.
Further, in compound semiconductor devices, a protective film comprising a silicon nitride film or silicon oxide film or the like is frequently formed on the principal surface of the substrate where the elements exist. The protective film serves to protect the elements from contamination and suppress the production of surface charge.
Therefore, when forming a metal electrode in a semiconductor device with a protective film, the protective film is required to be removed.
The process of forming a metal electrode of a conventional commonly known semiconductor device with a protective film will be described hereinbelow with reference to FIGS. 5A to 5E.
First, as shown in FIG. 5A, a substrate 102 having formed on its surface a protective film 100 is prepared.
The structure shown in FIG. 5B is then formed as follows. A photoresist is applied onto the protective film 100. Thereafter, by performing exposure and development, the photoresist layer 104 is removed at its portion where a metal electrode 112 is to be formed. The portion is a planned electrode formation region 102c of the photoresist. As a result, a hole 106 that reaches the protective film 100 is formed in the photoresist layer 104. Here, the opening in the surface of the photoresist layer 104 of the hole 106 is shown by a reference numeral 106a. Further, the planned electrode formation region 102c represents a region with the same planar shape as the opening 106a on the principal surface 102a of the substrate 102.
The structure shown in FIG. 5C is formed next. That is, the exposed portion of the protective film is etched by means of the RIE (reactive ion etching) method with the photoresist layer 104 serving as a mask. By the way, in the etching process, a component of the plasma particles penetrating the interior of the hole 106, flies obliquely toward the principal surface 102a of the substrate 102. Accordingly, the plasma particles strike onto the surface of the protective film 100 over a wider region than the planned electrode formation region 102c. As a result, the protective film 100 is removed through side etching from not only the region corresponding with the planned electrode formation region 102c but also as far as the peripheral region. As a result, within the hole 106, the protective film 100 is removed to form an exposure region 108 via which the principal surface 102a of the substrate 102 is exposed. For the reason provided above, the exposure region 108 has a larger surface area than that of the planned electrode formation region 102c. 
Thereafter, the structure shown in FIG. 5D is formed. That is, a metal film 110 is deposited through vacuum deposition over the whole face on the side of the principal surface 102a in a state where the photoresist layer 104 still remains as shown in FIG. 5C. Further, metal atoms that fly from the evaporation source have stronger directivity than the plasma particles mentioned earlier. Accordingly, the metal atoms that penetrate inside the hole 106 after passing through the opening 106a are deposited on a limited region of the exposure region 108. Specifically, metal atoms are not deposited over the whole of the exposure region 108 but are rather deposited in a cross-sectional trapezoidal shape in a slightly wider region than the planned electrode formation region 102c. Here, the metal film that is deposited in the planned electrode formation region 102c is shown by a reference numeral 110a. Further, the metal film that is deposited on the photoresist layer 104 is shown by a reference numeral 110b. 
Finally, the metal electrode 112 (metal film 110a) shown in FIG. 5E is obtained. That is, unnecessary portion to the metal film 110b on the photoresist layer 104 is removed by dissolving the photoresist layer 104 by means of an organic solvent or the like.
As shown in FIG. 5E, an uncoated region 114 via which the principal surface 102a of the substrate 102 is exposed exists between the metal electrode 112 and the protective film 100 that surrounds the metal electrode 112. This is because, as shown in FIG. 5C, excessive side etching occurs as a result of etching performed by means of the RIE method and hence the protective film 100 is removed over a wider region (exposure region 108) than the planned electrode formation region 102c. 
The uncoated region 114 is not coated by the protective film 100 or metal electrode 112 and is therefore readily contaminated, which causes the production of surface charge.
Hence, a technique that does not allow the uncoated region 114 to be produced, that is, a technique that suppresses side etching of the protective film 100 (FIG. 5C) is desirable.
According to the technique, a porous silica film is changed to a hardly etchable film by exposing the side of the to be etched film comprising the porous silica film by means of hydrogen plasma. A conventional technique for suppressing side etching is known from Japanese Laid-Open Patent Application No. 2005-45176 (corresponding to U.S. Patent Application Publication No. US 2005/0017364 A1).
However, the conventional technique necessitates additional hydrogen plasma processing in order to suppress the side etching of the porous silica film. By this reason, the conventional technique has the problem that the number of steps of the etching processing increases.